1. Field of the Invention
The present invention relates to notch antennas having multiple resonant frequency bands and wireless devices using the same.
2. Description of the Related Art
With the current trend toward multifunctional mobile wireless terminals, various functions, such as GPS (Global Positioning System) and Bluetooth™ (a short range radio communication system), are implemented in mobile wireless terminals. In addition, mobile wireless terminals each incorporating a plurality of wireless communication systems using different frequency bands, or what is called multiband mobile wireless terminals, have been developed and have appeared on the market.
From the viewpoint of downsizing and weight reduction of wireless terminals each employing a plurality of wireless communication systems, it is preferable to share a built-in antenna among the plurality of wireless communication systems.
Japanese Patent No. 3916068 discloses a notch antenna provided by forming a notch (or a slit), which is a slim groove having an open end, in a ground plate (ground conductor). The notch antenna is relatively simple in structure and therefore suitable as an antenna device built in a mobile or small wireless device.
The resonant frequency of a notch antenna is determined by the length of the slit, and the length is usually set to approximately 0.2 times the wavelength of a working frequency. For working frequency bands used by PDC (Personal Digital Cellular) systems (approximately 800 MHz) or GSM (Global System for Mobile communications) systems (approximately 800 to 900 MHz) in the past, the slit is usually set as long as 70 to 80 mm, and it is not easy to incorporate such a long-slit notch antenna in mobile phone terminals supporting these systems. On the other hand, the notch antennas can be easily adapted to third generation mobile phone systems (e.g., W-CDMA system operating at approximately 2 GHz), GPS-installed mobile phone terminals (approximately 1.575 MHz) and systems operating in higher frequency bands, such as Bluetooth (2.5 GHz).
Proposed in Japanese Patent No. 3844717 and Japanese Unexamined Patent Application Publication No. 2004-274445 are notch antennas in which a plurality of slits are formed in a ground plate to obtain multiband operation.
Japanese Unexamined Patent Application Publication No. 2004-32303 proposes a technique of producing resonance in a plurality of frequency bands by providing a resonator on a short-circuited end (closed end) side of a slit of a notch antenna to make the slit behave as if it is short at high frequencies.
Moreover, Japanese Unexamined Patent Application Publication No. 2004-336328 proposes a technique of obtaining broadband characteristics (double resonance characteristics) by inserting a parallel resonant circuit in the vicinity of a closed end of a slit in parallel, the parallel resonant circuit including a capacitor arranged in parallel with inductance occurring between the feed point and short-circuited end.